Optical Density and Light Speed. Refraction is the bending of the path of a light wave as it passes from one material to another material. The refraction occurs at the boundary and is caused by a change in the speed of the light wave upon crossing the boundary. The tendency of a ray of light to bend one direction or another is dependent upon whether the light wave speeds up or slows down upon crossing the boundary. Because a major focus of our study will be upon the direction of bending, it will be important to understand the factors that affect the speed at which a light wave is transported through a medium. Light Propagation Through a Medium The mechanism by which a light wave is transported through a medium occurs in a manner that is similar to the way that any other wave is transported - by particle-to-particle interaction.
In Unit 10 of The Physics Classroom Tutorial, the particle-to-particle interaction mechanism by which a mechanical wave transports energy was discussed in detail. Look it Up! Concepts and Formulas | Depth of Field/Focus. Depth of Field and Depth of Focus When considering resolution in optical microscopy, a majority of the emphasis is placed on point-to-point lateral resolution in the plane perpendicular to the optical axis (Figure 1).
Another important aspect to resolution is the axial (or longitudinal) resolving power of an objective, which is measured parallel to the optical axis and is most often referred to as depth of field. Axial resolution, like horizontal resolution, is determined only by the numerical aperture of the objective (Figure 2), with the eyepiece merely magnifying the details resolved and projected into the intermediate image plane. Just as in classical photography, depth of field is determined by the distance from the nearest object plane in focus to that of the farthest plane also simultaneously in focus. This interchange of nomenclature can lead to confusion, especially when the terms are both used specifically to denote depth of field in microscope objectives.
Table 1. Numerical Aperture and Resolution. What is resolution and what does it have to do with the numerical aperture number of an objective lens (or a condenser lens, for that matter)? Resolution can be defined as the ability of a microscope to allow one to distinguish between small objects. In other words, how crisp and sharp is an image at any given magnification? The numerical aperture number is directly related to the cone of light from the specimen at its vertex which is brought into the lens.
Simply put, when light hits an object, it diffracts. A single beam of light will be split into several different diffraction orders bent at increasing angles from the original impinging beam. The second advantage of using a higher numerical aperture is that since more orders of diffraction from the object are brought into the lens, more light generally is brought into a higher numerical aperture lens producing brighter images. The following diagram shows what happens to the Airy disk with increasing numerical aperture. Lens_equation.